1. Field of the Invention
The present invention relates to a semiconductor device, and, more particularly, to a semiconductor device that includes fast, highly-integrated and highly-reliable memory using memory cells which store information using a change in magnetoresistance.
2. Description of the Prior Art
A magnetoresistive random access memory (MRAM) has been developed as a memory which is not limited to the number of read/write operations while it is a non-volatile memory.
An MRAM stores information by utilizing such a magnetoresistive effect that the resistance of an element varies depending on the direction of magnetization of a ferromagnetic material in a memory cell. Recently, attempts have been made to development magnetic tunnel junction (MTJ) elements which have a greater magnetoresistive ratio called magnetoresistance (MR) than elements of the prior art and to apply the MTJ elements to MRAMs. The attempts have led to a possibility of realizing a memory which performs as fast a read-out operation as a static random access memory (SRAM) and whose integration scale is as large as that of a dynamic random access memory (DRAM).
For example, such memories are described in IEEE International Solid-State Circuits Conference, DIGEST of TECHNICAL PAPERS, pp. 128-129 and pp. 130-131, 2000.
FIG. 2 shows the basic structure of memory cells used in the memories. Each memory cell comprises a single MTJ element MTJ and a single transistor MMC, which are connected to a write word line WW, a read word line WR and a data line DL.
The MTJ element MTJ has such a structure as to have a tunnel insulating film sandwiched between a fixed layer of a ferromagnetic material whose direction of magnetization is fixed in a normal operation and a free layer of a ferromagnetic material whose direction of magnetization can be flipped by a read-out operation. The resistance across the two ends of the MTJ element changes in accordance with the directions of the magnetization of the two ferromagnetic layers. The resistance is low when both directions of the magnetization are the same, whereas the resistance is high when they are opposite to each other.
A read-out operation is executed as illustrated in FIG. 3. The read word line WR is selected to enable the transistor MMC, a voltage is applied across the ends of the MTJ element MTJ and stored information is read out by detecting a current IDL which flows through the data line DL according to the magnetoresistance of the MTJ element MTJ.
A write operation is executed as illustrated in FIG. 4. Specifically, with a current IWW of the selected write word line WW serving as a write word line current IWS, the write operation is carried out by letting a write current ID1 or ID0 corresponding to write data “1” or “0” flow through the data line DL.
At this time, a magnetoresistive ratio MR or the ratio of an increase in resistance in the high resistance state to that in the low resistance state shows a hysteresis characteristic as shown in FIG. 5. The hard axis magnetic field that is produced by the write word line current IWS makes it easier to flip the magnetization of the MTJ element, thus providing a narrow hysteresis characteristic with respect to the data line current IDL that produces an easy axis magnetic field. This makes it possible to write stored information by flipping the magnetization of only the memory cell that is selected by the write word line WW.
To make the practical use of an MRAM using the memory cells as shown in FIG. 2, the high S/N ratio is important. As shown in FIG. 3, a read signal from a memory cell is the data line current IDL according to the invention the magnetoresistance of the MTJ element. As described in the aforementioned documents, the magnetoresistive ratio MR of the MTJ element is as high as several tens of %. Further, as shown in FIG. 6, the magnetoresistive ratio MR is highly dependent on the applied voltage. For example, whereas the magnetoresistive ratio MR0 in the vicinity of the applied voltage of 0 V is 40%, the applied voltage, VH, that provides a magnetoresistive ratio of 20%, a half the magnetoresistive ratio MR0, is about 0.3 V. Therefore, the difference between signal currents for write data of “0” and “1” has a peak value with respect to the applied voltage and the signal current cannot be increased by increasing the applied voltage.